Biofouling is the undesirable accumulation of organisms including bacteria, plants, algae, and animals on submerged structures. Biofouling includes both microfouling (biofilm formation and bacterial adhesion) and macrofouling (attachment of larger organisms). Biofouling organisms are classified as hard or soft fouling types. Hard (calcareous) fouling organisms include barnacles, encrusting bryozoans, mollusks such as oysters, polycheate and other tube worms, zebra mussels and tunicates. For instance, the fully grown barnacle is a stationary calcareous arthropod characterized by a cone shell enclosing layers of calcinous plates. The mechanical strength of the barnacle's attachment is very high, and it is difficult to mechanically remove them from marine surfaces. Soft (non-calcareous) fouling organisms include seaweed, hydroids, algae and biofilm “slime”. Successful removal of soft fouling organisms from marine surfaces also presents many difficulties.
Biofouling on aquatic vessels poses significant operational and safety issues. In some instances, the hull structure and propulsion systems can be damaged. Over time, the accumulation of biofouling materials on hulls can increase both the hydrodynamic volume of a vessel and the frictional effects leading to an increased drag, estimated to be up to 60% increase for a biofouled surface. The drag increase has been seen to decrease speeds by up to 10%, which can lead to increased fuel consumption. With fuel typically comprising up to half of aquatic transport costs, improved antifouling methods and materials could save the shipping industry billions of dollars each year. For example, the control of biofouling accumulation has become the single most expensive maintenance problem incurred by the U.S. Navy for ship operations.
Aquatic vessels are not the only surfaces subject to biofouling. Since biofouling can occur almost anywhere water is present biofouling poses risks to a wide variety of both fresh and saltwater submersible structures and presents significant cost issues to entire industries including paper manufacturing, food processing (aquaculture), underwater construction, oil and gas recovery, and desalination.
Historically, aquatic coatings (e.g., marine paints) have included biocides designed to leach from the coating over time to thereby prevent biofouling by virtue of the biocide's toxicity to biofouling organisms. Commonly used biocides have included certain metals and their salts. Two particular compounds, tributyltin (TBT) and cuprous oxide have been in commercial use for decades as antifouling agents in marine paints. However, TBT is now banned worldwide due to high toxicity of the leachate to free-floating organisms. Cuprous oxide is also of concern because of the build-up of leached copper in harbor sediment. Furthermore, broad spectrum leached biocides cannot be used in the aquaculture industry at all, as this industry requires agents that will not adversely affect the growth of the farmed fish themselves and which do not provide danger to the ultimate consumer.
Unfortunately, attempts to replace the traditional biocides with alternatives that are non-toxic when released into the surrounding water, or mechanical alternatives that could dislodge attaching marine growths, have met with limited success. For instance, self-polishing polymer coatings such as silicones and fluoropolymers (e.g., Teflon™) have offered possible alternatives to marine paints containing toxic antifouling agents. These coatings can have a low surface energy leading to low attachment capability of marine organisms. Unfortunately, however, these materials still require some sort of biocide additive to successfully prevent biofouling and hydrodynamic pressure still needs to be applied to dislodge the biofouling organism, i.e., merely the low surface energy surface is not enough to prevent biofouling.
Non-toxic surface-bound alternatives to leaching biocides have also been examined. For instance, it has been found that noradrenaline, a catecholamine with biological roles including as a hormone and a neurotransmitter, deters fouling marine invertebrates from settling when covalently bound to a surface or included as part of a coating which coats and covers a surface, thus preventing biofouling. Despite its antifouling effectiveness in lab studies, natural noradrenaline is not an ideal molecule for inclusion as an active agent in marine antifouling paints due to the fact that it has a short half-life (minutes) and it spontaneously oxidizes into adrenochrome, thus resulting in a loss of biological activity.
What are needed in the art are antifouling compounds that can be incorporated into coatings or otherwise applied on aquatic surfaces that can afford maximum protection from biofouling without harming the local environment.